KANDAHAR, Afghanistan – A violent resurgence of Taliban and al-Qaeda fighters in Afghanistan is putting U.S. soldiers and international aid workers at increasing risk, especially in the southern part of the country, the back yard and birthplace of the Taliban.

U.S. military patrols are coming under attack frequently. Highway construction workers – foreigners and Afghans – have had their throats slit for tampering with Taliban flags placed along roadsides.

Nearly three years after the U.S. military toppled the Taliban government in Afghanistan, it is increasingly obvious that the military effort has failed to fully defeat the enemy, which has regrouped and now threatens efforts to create a stable government.

Election workers are being abducted, shot and beheaded. Voter-registration sites are being bombed, even when they are located at mosques. Now the Taliban has begun delivering ominous "night letters" – notes slipped to local officials warning them not to help with upcoming elections.

The country's historic first presidential election is scheduled for Oct. 9 – three months later than originally planned because of security worries – and parliamentary elections have been pushed back until April.

In a shocking announcement Wednesday, Doctors Without Borders said it is withdrawing from Afghanistan after nearly 25 years there, including service throughout the Soviet occupation of the 1980s and the subsequent Taliban rule. Doctors Without Borders won the 1999 Nobel Peace Prize for its work in some of the world's most deprived and dangerous places.

The group said the Afghan government has identified the men who killed five of its staffers in June but refuses to apprehend them for political reasons. The government's failure to act "sends the message that it's acceptable to kill aid workers," said operations director Kenny Gluck.

Taliban and militia groups continue to threaten Doctors Without Borders and other aid agencies. U.N. projects in the south have been aborted because of the threats and attacks.

The United Nations refuses to re-enter the region until U.S.-led coalition forces can establish better security there. A multinational force of 17,000 troops is deployed in Afghanistan, including 6,600 U.S. combat forces. The fledgling Afghan National Army still is being trained and equipped.

U.S. and NATO forces have a strong presence in Kabul, the Afghan capital, but their grip on the rest of the country is tenuous. Hamid Karzai, the U.S.-backed president, is derisively said to be little more than the mayor of Kabul.

Much of the rest of Afghanistan is controlled by powerful warlords who command large private militias. They are largely financed by a booming export trade in opium and heroin.

In southern and eastern Afghanistan, where Osama bin Laden and Taliban leader Mullah Omar are believed to be hiding, the Taliban effectively controls large swaths of territory. U.S. military officials say foreign Islamist fighters and al-Qaeda financiers are aiding the Taliban.

"These guys are coming over from Pakistan, migrating into our districts (in the south)," said Lt. Col. Mike McMahon, an Army commander at Kandahar Air Field. "They come across in groups of 10 or 15."